1. Field of the Invention
This invention relates generally to implantable cardiac stimulators, and more particularly to the circuit modules encased within cardiac stimulators.
2. Description of the Related Art
The advent of implantable cardiac stimulation systems, such as pacemakers and defibrillators, has brought welcome relief to many patients suffering from various forms of cardiac arrhythmia. Conventional cardiac stimulator systems typically consist of a cardiac stimulator and one or more elongated leads. The cardiac stimulator may be a pacemaker, a defibrillator, a sensing instrument, or some combination thereof. The circuitry, batteries, and other components of the cardiac stimulator are ordinarily encased within a metallic housing commonly referred to as a "can." Most of the electronic components for the cardiac stimulator are mounted on a small circuit board commonly known as a multi-chip module (hereinafter "MCM").
The proximal ends of the leads of the cardiac stimulator system are connected physically and electrically to the cardiac stimulator via a structure commonly known as a header. The distal end of the lead is implanted near the site requiring electrical stimulation or sensing. The leads function to carry electrical signals from the cardiac stimulator to the targeted tissue and signals from the targeted tissue back to the cardiac stimulator.
For most implantable cardiac stimulators, implantation requires an incision in the right or left pectoral region above the areola and formation of a pocket in the subcutaneous tissue by blunt dissection. The leads are then passed into the body to the sites requiring electrical stimulation, usually with the aid of a stylet. The proximal ends of the leads are then connected to the header of the cardiac stimulator and the cardiac stimulator is inserted through the incision and placed in the pocket. The incision is then closed by conventional suturing. The post-operative appearance of the implant area will depend to a large degree on the size of the cardiac stimulator.
The first implantable cardiac stimulators were relatively bulky devices requiring large tissue pockets for implantation. The size of successive generations of implantable cardiac stimulators has diminished dramatically, particularly in the past ten years. Much of this advance in miniaturization is due to advances in the design of semiconductor circuits and fabrication processes.
Size reduction of the internal circuitry of cardiac stimulators has always been an important design goal for engineers. The reasons are at least two-fold. Smaller MCMs can be housed in smaller cans. Smaller cans translate into smaller surgical incisions, smaller subcutaneous pockets, and smoother postoperative skin surfaces. Miniaturization in circuitry also translates into higher device density on the MCM. The result is that more circuitry may be packed into the same or a smaller space. This has to the development of much more sophisticated cardiac stimulators that can perform many different types of useful therapeutic functions, and programmed and reprogrammed to match the patient's condition.
A conventional MCM module consists of a number of electronic devices disposed on one or both sides of a flat insulating substrate. Depending upon the type of cardiac stimulator, the devices may be discreet devices, such as resistors and capacitors, or more highly integrated devices, such as power transistors, microprocessors, telemetry circuits, or induction coils for rechargeable storage devices. These integrated circuits are normally fabricated on a semiconductor chip or die. Many types of electronic devices, such as microprocessors, are commonly disposed in a semiconductor package, such as a PLCC, CERDIP or other type of package that is pin soldered or surface mounted to the MCM substrate. Other devices, such as power transistors, are commonly connected to the MCM substrate as bare die using the well known chip-and-wire technique. In this technique, the bare die are surface mounted to the MCM substrate and bonding wires are connected to the bond pads of the die and to the conductor traces on the substrate by thermal sonic or thermal pressure bonding techniques. Separate space on the MCM substrate must be allotted for each die.
Conventional chip-and-wire processing has certain disadvantages. To begin with, surface mount processing and chip-and-wire processing are generally incompatible. The problem stems from the fact that the solder and solder flux used to mount a surface mount package may short or damage the tiny bonding wires connecting the bonding pads of a bare die to the MCM substrate. One method currently used to alleviate the incompatibility problem involves mounting bare die on one side of the MCM substrate using chip-and-wire processing, and surface mounting other components on the opposite side of the MCM substrate. This method requires complex metallization on both sides of the MCM substrate. In addition, chip-and-wire technology requires separate space on the MCM substrate for each component. This technique limits the potential miniaturization of cardiac stimulators and does not free up space on the MCM substrate that could otherwise be used to incorporate additional useful electronic components into the MCM. Finally, each chip-and-wire mounted component must be separately tested. Such component-by-component electrical testing adds to cost of the overall manufacturing process.
The present invention is directed to overcoming or reducing one or more of the foregoing disadvantages.